1. Technical Field
The invention is related to the general field of chemical-mechanical polishing compositions, and to the specific field of compositions for polishing and planarization during semiconductor fabrication.
2. Description Of The Related Art
A contemporary process of manufacturing semiconductor devices begins with a thin circular disc of silicon or other semi-conductive material, which forms a substrate upon which many individual semiconductor devices are fabricated. Such discs range in diameter from one inch to twelve or more inches, with the most common standard sizes being of five, six or eight inch diameters.
The fabrication involves the creation of minuscule integrated circuity in various alternating layers formed on the substrate disc. As layers are added to the substrate, it is common to refer to the built-up disc as a "wafer", and that terminology will be used in this description. To understand the role of chemical-mechanical polishing and planarization in this fabrication of wafers, it is sufficient to consider only the formation of an initial set of dielectric and conducting layers, with the understanding that complex devices may have many layers.
The silicon disc is coated with a layer of dielectric material, the more common materials being silicon oxides such as silicon dioxide (SiO.sub.2), BPSG (boron phosphorous doped silicon glass), SOG (spun-on glass), TESO (tetraethylorthosilicate), and PETEOS (plasma enhanced tetraethylorthosilicate). Photolithography equipment is then used to create a circuitry pattern on the oxide layer. Precise application of the resist patterning is necessary for high-density micro-circuitry, and it is well known that the precision of current photolithography equipment is dependant upon the degree to which the oxide layer can be made flat (to provide uniform depth of focus and reduce other variables). Thus, various techniques have been devised for depositing the oxide layer evenly, and for etching or polishing the surface of the oxide layer to approach flatness or planarity. This invention is related to the polishing technique known generally as chemical-mechanical polishing ("CMP"). The term "polishing" as used herein, is intended to include planarization.
Chemical-mechanical polishing of semiconductor wafers at a commercial production rate is generally accomplished with rotational, linear or orbital polishing machines in which a wafer is pressed against a polish pad mounted on a moving platen in the presence of a CMP composition, or "slurry", between the pad and the wafer surface. The CMP slurry contains a chemical agent which is corrosive to the material to be removed, at a concentration which produces either an acidic or basic solution of sufficient pH to cause controlled surface dissolution on the material, and some type of abrasive particles to mechanically remove material from the surface. The combination of chemical and mechanical removal can generally produce more even removal at a higher rate than chemical etching or abrasive polishing alone. In addition, the CMP process produces a more "global" planarization across the wafer as opposed to the more local planarization of the chemical etch processes.
In addition to chemical-mechanical polishing to achieve uniform flatness in dielectric oxide layers, CMP compositions may be used for global planarization of a metal layer or layers on a semiconductor wafer. Metal layers (tungsten, copper, aluminum or various alloys) or metal-containing layers are applied over the patterned dielectric layer to form conductive regions in the fabrication of the semiconductor wafer. For example, the first dielectric layer described above may have been etched after photolithographic patterning to create patterns of holes and trenches corresponding to conductive interconnects and vias for that level in the semiconductor device. These holes and trenches will be filled with conductive material, usually by deposition of a metal or metal-containing layer or layers over the patterned dielectric. As an example of metal layering, the conductive material may be tungsten (W), which does not adhere strongly to silicon dioxide. To achieve better adherence, a thin seed layer (500-1000 angstroms) of titanium (Ti) is first applied to the patterned dielectric, then a second thin layer (500-1000 angstroms) of titanium nitride (TiN) is applied over the Ti layer to facilitate the bonding of the thicker tungsten layer.
The deposition of these metal layers involves covering the surface of the dielectric, not just filling the holes and trenches. To achieve complete filling of the holes and trenches with conducting material, it is usually necessary to deposit a relatively thick metal layer. However, to isolate conductivity in the wafer to the intended circuit patterns, it is necessary to remove the primary metal layer and any underlying seed and bonding metal layers until the wafer surface is free from metal except for the intended circuitry pattern. Then, a new layer can be deposited on the wafer. However, while removing the W/TiN/Ti metal layers, the CMP process should not erode too much of the exposed dielectric. The removal of the metal layers except for the circuitry pattern, and the leveling of the dielectric and metal surfaces, is often referred to as global planarization of the wafer. The ability of the CMP process to remove the higher regions of metal without removing an excessive amount of exposed lower regions of dielectric is often described as a "selectivity" of a CMP process. A CMP process may also demonstrate a selectivity for particular materials over other materials.
Many of the variables effecting CMP are known, and can be altered and balanced to achieve a desired level of performance for a particular polishing task or particular polishing tool. Changing any one of the factors to increase one aspect of performance often requires compromise in other areas of performance. For example, polishing rate can usually be enhanced by increasing down pressure and platen speed, but at the compromise of uniformity, surface defects and the potential for subsurface damage. Similarly, more aggressive pH levels and more abrasive particles in the slurry may increase removal rate at the compromise of planarization efficiency, selectivity, and the creation of scratches.
A principle objective of this invention is to improve the CMP slurries by the use of unique solid particles. Consequently, this background section will concentrate primarily on the properties of prior abrasive and activating solid particles, and on the effects previously attributed to particle parameters, with the understanding that the other variables must be considered when setting up any particular CMP process. However, the properties and performance of these unique solid particles are so different from particles made of the same precurser materials by other methods that most CMP applications would likely be improved by using these new particles.
As a general rule of thumb, it is considered that abrasives made of harder substances remove material faster than abrasives of softer material, all other factors being equal. However, it is also believed that harder abrasives are more prone to cause uneven removal at a given down pressure, and will increase the degree of surface scratching. Consequently, it has been somewhat common to select softer abrasive material such as silica, ceria, and germania to polish the dielectric oxide layers, and harder abrasives to planarize hard metal layers such as tungsten and titanium. In contrast, CMP slurries using the unique abrasive particles as described in this invention should enable the use of harder abrasive materials to achieve high removal rates even on oxides and softer metal layers such as copper and aluminum, without sacrificing uniformity of removal, selectivity, and freedom from scratching.
Similarly, it has been conventional wisdom that increasing the percentage weight of solids in the slurry increases removal rate, but that increasing the percentage of abrasive material will lower the selectivity between high and low regions. CMP slurries using these unique particles should enable the use of comparatively higher percentage weights of solids to achieve rapid removal rates without sacrificing selectivity.
There is some empirical data and theory that abrasive particle specific surface area is a significant factor in CMP polishing, and that decreasing specific surface area improves planarization efficiency and uniformity of removal. (See, Effect of Oxide CMP Slurry Properties on Polishing Performance, M. Borba et.al.). Since it was believed that the way to decrease surface area of the solid material is to increase the primary particle size, it was postulated that the trade-off for uniformity required a choice between a slurry with an increased weight percentage of large particles or a slurry with a lower weight percentage of small particles. In contrast, the unique particles described for use in this invention have such low porosity and such generally uniform shape that slurries having very low abrasive surface area can be obtained with a large number of very small particles.
Some abrasive particles are known to have a high degree of chemical activity toward certain substrate materials, such as cerium oxide (CeO.sub.2) to silicon dioxide and other dielectric oxides. If such active material is used as the sole abrasive material in CMP slurry, high removal rate is achieved at the sacrifice of selectivity because the chemical dissolution takes place on all exposed portions of the substrate, regardless of relative height. Consequently, it has been attempted to use both CeO.sub.2 particles and SiO.sub.2 particles in a CMP slurry, but was found impractical because conventional CeO.sub.2 particles could not be formed which match the size and shape of the extremely small SiO.sub.2 particles used for critical polishing such as in semiconductor wafer fabrication. (See the discussion in the background section of U.S. Pat. No. 5,480,476, Cook et al). In contrast, the unique abrasive particles in this invention can be formed of CeO.sub.2 with sizes equal to or smaller than the finest SiO.sub.2 particles, and with a uniform shape and specific surface area similar to the finest SiO.sub.2 particles.
It has also been known that regardless of how small the primary particles of abrasive can be made by conventional fuming processes, the primary particles tend to fuse together into irreversible branched chains called "aggregates", and that the larger order aggregates must be removed from the slurry in some manner to avoid scratching or other surface imperfections in the polishing process. In contrast, these unique abrasive particles do not fuse together to form aggregates, but rather remain as primary particles of generally uniform shape, and thus do not create the hazard of aggregate scratching.
These and other advantages of the invention should become more apparent from reading the summary and the detailed description of the invention which follow. However, these sections are intended to explain one or more ways or processes for making and using the invention to persons skilled in the art, including the best mode that is known to the inventor at this time. They are not intended, nor should they be used, to limit the scope of the invention. It is very likely that additional CMP slurries using one or more aspects of the invention will be developed for various polishing applications.